Trace ions in the presence of matrix ions in environmental samples are commonly analyzed by ion chromatography. Typically, the detection of trace ions is enhanced by pre-concentration or by injecting a large volume of the sample designated as “large loop” injection. In addition to concentrating the species of interest, these approaches also focus or concentrate the matrix ions which may not be desirable in some cases. With the pre-concentration approach, there may be a loss of recovery of trace ions of interest, since a large matrix ion concentration tends to act as an eluent within the concentrator column. In some instances the presence of high levels and varying levels of matrix ions can affect the overall chromatographic performance and can cause higher variances in retention time and peak response reproducibility. In addition, high levels of matrix ions can cause loss of peak efficiency and resolution and decrease in column lifetime. Therefore, there is a need for a reliable ion chromatographic method that allows detection of trace ions in the presence of matrix ions.
Several different approaches have been discussed in the literature to minimize the levels of matrix ions such as pre-treating the sample with a pretreatment column, neutralizing the matrix ions using a neutralizer, and dialysis. The most common approach is to use a column to pre-treat the sample prior to separation to eliminate or to decrease the matrix ion concentration. For example, when the matrix ion consists of chloride, a silver form cation exchanger can be used to precipitate the chloride and thus facilitating the detection of other trace anions in the sample. The above pretreatment or matrix elimination step is either performed inline or offline. The offline steps are cumbersome and time consuming. Inline methods are preferred since they automate the analysis. However, typically the exchanger column is either discarded or reused after regeneration, thus adding to analysis costs and/or additional processing steps. It would be useful to truly automate the analysis.
One way to accomplishing this is by pursuing a two dimensional analysis. Online multidimensional chromatography has been used for analyzing complex samples. In some instances, the approach has been designated as column switching. The approach relies on segmenting the sample analysis chromatogram into manageable portions for further analysis. In the two dimensional approach, the matrix ions could be diverted to waste while the analytes of interest flow to a second column or dimension for further analysis or routed back into the primary column for further analysis. In some cases, the separations were accomplished using a single analytical pump. In some instances, two systems were used for the purpose of analyzing species of interest. The use of a suppressor differentiates two dimensional liquid chromatographic analyses from two dimensional suppressed ion chromatography analysis. The suppressor converts the eluent to the weakly dissociated form. With hydroxide eluents, the suppressed eluent is water which provides a good carrier liquid for concentrating species of interest by focusing onto a concentrator column. The focusing effect of the concentrator column enables analysis without substantial degradation of peak shapes. Although the prior art literature shows multiple columns with multiple chemistries and capacities, there were no means provided for improving the sensitivity of the ions of interest.
The literature describes a number of two dimensional separations. For example, Steven R. Villase{hacek over (n)}or (Anal. Chem 63, (1991), 1362-1366 disclosed a matrix elimination technique using heart cut and column switching method for the analysis of sulfite in analgesic formulation. The approach does not use a concentrator column or means for concentrating the heart cut. Additionally a suppressor was not used in this work. The pre-column and the analytical column were operated at a single flow rate.
Umile and Huber (J. of Chromatography, 723, (1996) 11-17) disclose a column switching technique for relative analyte enrichment. The concept is to divert the ions of interest into a second column and obtain improved resolution of peaks of interest. The approach uses a column C1 to run the first part of the separation and cutting the required heart cut into a second column C2. The analysis continues on C2 at the same flow rate to yield higher resolution separations of peaks of interest. This type of approach is not suitable for achieving sensitive detection of ions in the presence of matrix components.
Utzman and Campbell (LC-GC volume 9, 4, 301-302) describe a column switching method to pursue analysis of white liquors and dilute recirculating liquors. The approach uses a guard and an analytical column between which the flow is redirected. Chloride, sulfate and sulfite ions pass through the guard column and then the flow is switched to flow through the analytical column first followed by the guard column. This schematic allowed highly retained thiosulfate ion to elute at a reasonable run time of 9 minutes as opposed to 30 minutes in the standard method without valve switching. The approach uses a fixed flow rate.
Another variation of the above approach is shown by Columbini et. al. (J. Chromatography, A 822, (1998) 162-166) to analyze total nitrogen and phosphorus by switching the column flow with the matrix ions to waste using one pump and pursing the analysis of species of interest using a second pump. An overall reduction in run time along with improved resolution of species of interest was achieved. This approach also used a fixed flow rate.
Galceran and Diez (J. Chromatography, A, 675, (1994) 141-147 show the analysis of phosphate in samples containing high levels of sulfate. The approach used a column switching setup that allowed transfer of analytes of interest from one dimension to another dimension. No suppressors were used in this work. The above approach the two dimensions were operated with the same flow rate.
Rey et. al. (J. Chromatography A, 789 (1997) 149-155) discloses a column switching means for pursuing analysis of cations. The separation of trace inorganic cations in the presence of large concentration of sodium or ammonia was shown. In this approach there is no preconcentration and the divalent cations elute before the monovalent cations. Also a single pump is used in the analysis and the columns are operated at a single flow rate.
Peldszus et. al. (J. Chromatography, A, 793, (1998) 198-203 describes a two step column switching approach. In the first stage, the ion of interest oxalate was focused on to a concentrator column while the other ions are sent to waste. In the second stage the oxalate ion was analyzed using a different gradient method. A single pump was used in the analysis and the columns were operated at a single flow rate.
Huang et. al. (J. Liquid Chromatography, 22, 14, (1999) 2235-2245) shows the analysis of bromate in drinking water. They used a two stage process the first run was a matrix elimination run followed by analysis of bromate. The approach used a concentrator column to concentrate the heart cut after suppressing the eluent similar to the present invention. The analysis however was performed at a single flow rate. In contrast in the present invention the analysis was performed on a second dimension column of a lower volume and operated at the same linear flow rate or overall lower flow rate.
Bruno et. al. (J. Chromatography, A, 1003 (2003) 133-141) shows analysis of inorganic ions in waters of high salinity. The analytes of interest were diverted from the first dimension to the second dimension and then analyzed in the second dimension while the matrix ions were sent to waste in the first dimension. Both dimensions were operated at the same flow rates. In contrast, in the present invention the analysis was performed on a second dimension column of a lower volume and operated at the same linear flow rate or overall lower flow rate.
There is a need for a two dimensional method that provides enhanced sensitivity for ions of interest particularly when they are present along with matrix ions.